Jet mills are size reduction machines in which particles to be ground (feed particles) are entrained and accelerated in a stream or jet of fluid such as compressed air or steam, and then ground in a grinding chamber by their impact against each other or against a stationary surface in the grinding chamber. Different types of fluid energy mills can be categorized by their particular mode of operation. Mills may be distinguished by the location of feed particles with respect to incoming air. In the commercially available Majac jet pulverizer, produced by Majac Inc., particles are mixed with the incoming fluid before introduction into the grinding chamber. In the Majac mill, two streams of mixed particles and fluid are directed against each other within the grinding chamber to cause fracture of the particles. An alternative to the Majac mill configuration is to accelerate within the grinding chamber particles that are introduced from another source. An example of the latter is disclosed in U.S. Pat. No. 3,565,348 to Dickerson, et al., which shows a mill with an annular grinding chamber into which numerous fluid jets inject pressurized air tangentially.
During grinding, particles that have reached the desired size must be extracted while the remaining, coarser particles continue to be ground. Therefore, mills can also be distinguished by the method used to classify the particles. This classification process can be accomplished by the circulation of the fluid and particle mixture in the grinding chamber. For example, in “pancake” mills, the fluid is introduced around the periphery of a cylindrical grinding chamber, short in height relative to its diameter, inducing a vorticular flow within the chamber. Coarser particles tend to the periphery, where they are ground further, while finer particles migrate to the center of the chamber where they are drawn off into a collector outlet located within, or in proximity to, the grinding chamber.
Classification can also be accomplished by a separate classifier. Typically, this classifier is mechanical and features a rotating, vaned, cylindrical rotor. The air flow from the grinding chamber can only force particles below a certain size through the rotor against the centrifugal forces imposed by the rotation of the rotor. The size of the particles passed varies with the speed of the rotor; the faster the rotor, the smaller the particles. These particles become the mill product. Oversized particles are returned to the grinding chamber, typically by gravity.
Yet another type of fluid energy mill is the fluidized bed jet mill in which a plurality of fluid jets are mounted at the periphery of the grinding chamber and directed to a single point on the axis of the chamber. This apparatus fluidizes and circulates a bed of feed material that is continually introduced either from the top or bottom of the chamber. A grinding region is formed within the fluidized bed around the intersection of the fluid jet flows; the particles impinge against each other and are fragmented within this region. A mechanical classifier is mounted at the top of the grinding chamber between the top of the fluidized bed and the entrance to the collector outlet.
The primary operating cost of jet mills is from the power used to drive the compressors that supply the pressurized fluid. The efficiency with which a mill grinds a specified material to a certain size can be expressed in terms of the throughput of the mill in mass of finished material for a fixed amount of power produced by the expanding fluid. One mechanism proposed for enhancing grinding efficiency is the projection of particles against a plurality of fixed, planar surfaces, fracturing the particles upon impact with the surfaces. An example of this approach is disclosed in U.S. Pat. No. 4,059,231 to Neu, in which a plurality of impact bars with rectangular cross sections are disposed in parallel rows within a duct, perpendicular to the direction of flow through the duct. The particles entrained in the air stream or jet passing through the duct are fractured as they strike the impact bars. U.S. Pat. No. 4,089,472 to Siegel, et al. discloses an impact target formed of a plurality of planar impact plates of graduated sizes connected in spaced relation with central apertures through which a particle stream or jet can flow to reach successive plates. The impact target is interposed between two opposing fluid particle streams, such as in the grinding chamber of a Majac mill.
Although fluidized jet mills can be used to grind a variety of particles, they are particularly suited for grinding other materials, such as toners, used in electrostatographic reproducing processes. These toner materials can be used to form either two component developers, typically with a coarser powder of coated magnetic carrier material to provide charging and transport for the toner, or single component developers, in which the toner itself has sufficient magnetic and charging properties that carrier particles are not required.
The toners are typically melt compounded into sheets or pellets and processed in a hammer mill to a mean particle size of between about 400 to 800 microns. They are then ground in the fluid energy mill such as a fluidized bed jet mill or grinder to a mean particle size of between 3 and 30 microns. Such toners have a relatively low density, with a specific gravity of approximately 1.7 for single component and 1.1 for two component toner. They also have a low glass transition temperature, typically less than 70° C. The toner particles will tend to deform and agglomerate if the temperature of the grinding chamber exceeds the glass transition temperature.
In the fluidized bed jet mill or grinder, high velocity fluid, such as air is introduced through 3 to 5 air nozzle devices or nozzles located at the periphery of the grinding chamber and centrally focused. The high velocity air flow from these nozzles entrains and accelerates the particles of material towards the center of the mill. Size reduction is accomplished through the ensuing particle to particle collisions. This method of size reduction has been found to be most effective for size reduction of low-melt compounds typically found in current toner formulations.
In such toner production, size reduction is typically the rate limiting operation as well as having the highest process contribution to the manufacturing cost. Much effort has been concentrated on studying and understanding the size reduction process in order to increase its efficiency and thus maximize throughput rate at minimum cost.
Two factors, the probability of particle to particle collisions and the kinetic energy of these particles during such collisions are understood to affect the efficiency of the size reduction process.
Unfortunately however, fluidized bed jet mills or grinders which are used for such grinding or size reduction of toner particles, have an extremely low energy utilization efficiency. For example, it has been estimated that only 5% of total energy used up by a size reducing fluidized bed jet mill is actually utilized in particle size reduction. Such a low energy utilization efficiency is an opportunity for mill and/or nozzle designs to increase the energy efficiency of the process, thus resulting in significant operating cost savings.
Conventionally, several approaches, including nozzle redesigns have been tried, and continue to be tested for improving grinding energy utilization efficiency and throughput rate of such fluidized bed jet mills or grinders. Improved nozzle designs are directed towards increasing the probability of particle to particle collisions and towards increasing the kinetic energy of particle impacts.
A first type of conventional nozzle consists of a nozzle device having a single bell or flared profile opening or nozzle that discharges a single stream or jet of fluid and has a converging-diverging bell profile. The bell profile includes a converging region, a throat region, and a straight diverging flare region from the throat region to the discharge end.
Another type of conventional nozzle design as disclosed for example in U.S. Pat. No. 5,423,490 consists of a nozzle device having 4 small bell or flared profile openings or nozzles that each can discharge a small jet of fluid, for a total of four such jets. The four jets together then form a single composite jet downstream from the discharge end of the nozzle device. Thus this nozzle works on the concept of subdividing the main nozzle device into 4 smaller focused nozzles that provide the opportunity to entrain more material into the composite jet. As such, it is claimed that relative to the single bell or flared profile opening discharged stream nozzle device, this latter design allows for increased entrainment of particles of material being introduced into the individual fluid jets as they are being discharged from the 4 flared nozzles or openings.